The present invention relates to a method for distinguishing between a semi-soft magnetic materialand a soft magnetic material. The semi-soft material and/or the soft magnetic material may be used as a security feature in or on the substrate of a security article.
Soft magnetic security features are well known in the art of electronic article surveillance systems (EAS) and are often called anti-pilferage tags. The EAS systems make use of the non-linear magnetic properties of the B-H loop of the soft magnetic material. Small activating fields typically drive the soft magnetic material into saturation. Sensitivity to small fields is required here because it is difficult to generate a large magnetic field at a distance from a source, and typical EAS systems need to interrogate as large a volume as possible, e.g. the public access routes in and out of shops. The security features used here are therefore commonly based upon very soft magnetic materials such as the amorphous Metglas(copyright) or Vitrovac(copyright) or thin films such as made of a CoaFebNicModSieBfalloy, where a to f are atomic percentages and a ranges between 35% and 70%, b between 0% and 8%, c between 0% and 40%, d between 0% and 4%, e between 0% and 30%, f between 0% and 30%, with at least one element of each of the groups (b, c, d) and (e, f) being non zero. Such a CoaFebNicModSieBf composition is hereinafter referred to as a CoFeNiMoSiB composition. CoFeNiMoSiB films are marketed under the name of Atalante(copyright). The term xe2x80x9cthin xe2x80x9d here refers to a film having a thickness, which is smaller than 10 micrometer. These materials have a very low coercivity and a high magnetic permeability.
Within the context of the present invention, the terms xe2x80x9csoft magnetic material xe2x80x9d typically refer to materials having a low magnetic saturation field Hs, i.e. those materials require a magnetic field ranging between 3 A/m and 100 A/m (measured at 1 kHz) to saturate.
Using non-linear magnetic properties for the authentication of objects could also be an attractive approach because of simplicity and sensitivity. However, the approach would be of little use if the security elements set-off the alarms of the gates commonly used for EAS. The approach would also be of little use unless the security elements were difficult to obtain or copy.
Patent applications WO-A-98/26378 and WO-A-98/26377 disclose how to solve the above problem. The security element used comprises small, elongated magnetic particles which require a magnetic field greater than 100 A/m, and preferably greater than 300 A/m, to saturate. This property is chosen to ensure that the magnetic hardness of the particles is sufficiently high that they will not be driven into saturation at the field strengths commonly used in EAS gates. The security feature used here will therefore not set-off the alarm of the EAS gates.
In addition it is desirable to keep the magnetic field required for saturation well below that at which more commonly available Ferro-magnetic materials will saturate and to keep it at a sufficiently low level that the particles can be saturated, and therefore detected, at short ranges from a compact reading apparatus. In general this implies magnetic fields of less than about 3000 A/m.
Within the context of the present invention, the terms xe2x80x9csemi-soft magnetic material xe2x80x9d refer to magnetic materials typically having a magnetic saturation field Hs ranging from 100 A/m to 3000 A/m, e.g. from 200 A/m to 3000 A/m, preferably from 300 A/m to 3000 A/m (measured at 1 kHz).
Although the generation of high harmonics at low magnetic field strengths is particular to the soft magnetic materials in the case of EAS and to the semi-soft magnetic materials in the case of authentication, the inventors have discovered, however, that there is no clear difference between the harmonics generated from these types of materials. This is particularly true if the orientation of the security element is varied relative to the magnetic field.
Another problem with soft magnetic materials and semi-soft magnetic materials is that soft magnetic materials may be looked as semi-soft magnetic materials at a great distance between the drive coil and the material.
Moreover the drive field at which the security element will saturate, will vary with the orientation of the security element in the field. These problems can be solved by making the authentication method a contact one or by ensuring that the spatial orientation of the drive coil and material are fixed. However, for hand-held applications it is most convenient to validate the security element with a non-contact reading where the spatial orientation between drive coil and material is not fixed.
Still another problem is that there may be a magnetic field, external to the field generated by the drive coil, which could bias the total field.
EP-A1-0295085, EP-A2-0366335 and U.S. Pat. No. 5,204,526 all disclose magnetic material in the form of thin films or in the form of thin strips or wires used as markers or identifiers in detection or recognition systems. All documents suggest the use of magnetic material with two or more different coercive forces. These documents, however, are silent with respect to the difference between soft magnetic and semi-soft magnetic materials.
It is an object of the present invention to avoid the problems of the prior art.
It is a further object of the present invention to provide an authentication system, which can discriminate between various types of soft magnetic and semi-soft magnetic materials.
It is a further object of the present invention to provide a non-contact and a hand-held method for authentication.
It is also an object of the present invention to provide a compact low cost reading apparatus, which can be used to detect the special markers at distances up to a few centimeters.
According to the invention there is provided a method for distinguishing between a semi-soft magnetic material and soft magnetic material. The method comprises following steps:
(a) emitting an electromagnetic drive signal of one or more particular frequencies to an article so that any present semi-soft magnetic or soft magnetic material in the article go into saturation for both positive and negative magnetic fields;
(b) detecting an electromagnetic detection signal emanating from the article;
(c) measuring time or relative phase delays between one or more reference points of the drive signal and points at which positive and negative peaks of the detection signal occur;
(d) comparing the measured time or relative phase delays with values, which are typical for semi-soft magnetic material in order to make a decision whether the material is soft magnetic or semi-soft magnetic.
The method may comprise a further step of measuring the heights of the positive and negative peaks. The height of the peaks of the detection signal gives an indication about the distance or the orientation of the article. In a preferable embodiment only measurements which fall within a predetermined range of the heights are further processed. The time or relative phase delay between a reference point of the drive signal and a point at which the peaks occur give, together with the height of the peaks, an indication of the magnetic softness of the article.
Due to the fact that an indication is given about the distance or orientation of the material, the detection method can be a non-contact method, and more particularly a hand-held method. Within the context of the present invention, the terms xe2x80x9chand-held methodxe2x80x9d refer to the use of a small and light weigth detection apparatus with sizes not much greater than sizes of current available palm top organizers or portable telephones. A hand-held method is a method which can be applied outside a dedicated laboratory. The hand-held method can be applied everywhere, e.g. at the point of sales or point of transaction, in order to check magnetic security features in articles.
In a preferable embodiment of the invention, following steps occur:
(a) a first time or relative phase delay (A) between a first reference point of the drive signal current and the point at which a positive peak occurs is measured;
(b) a second time or relative phase (B) between a second reference point of the drive signal current and the point at which a negative peaks occurs is measured;
(c) the first and second time or relative phase delays (A and B) are summed.
The first reference point of the drive signal current may be equal to or different from the second reference point of the drive signal current. As will be explained hereafter, this sum A+B results in a reliable indication for the coercive force of the magnetic material used in the article and in a reliable indication whether the magnetic material is soft magnetic or semi-soft magnetic.
In a preferable embodiment the electromagnetic detection signal is proportional to the rate of change of magnetic flux density in the article (dB(t)/dt).
In another example the electromagnetic detection signal is proportional to an integral of the rate of change of magnetic flux density in the article (B(t)).
In a more elaborated example, the detection method further comprises a step of measuring the width of the peaks of the detection signal at one or more levels in order to discriminate semi-soft magnetic security features from Ferro-magnetic materials such as iron.
The magnetic material used as security feature can take many forms.
In a typical example the semi-soft magnetic security feature comprises a number of fibres such as disclosed in the above-mentioned patent applications WO-A-98/26378 and WO-A-98/26377.
In another example the semi-soft magnetic security feature comprises a thin semi-soft magnetic film.
In both examples the demagnetisation factor N of the fibres or the thin films is very low. Preferably, the demagnetisation factor N ranges from 1031 5 to 1031 2, e.g. from 1031 5 to 1031 3, preferably from 1031 5 to 1031 2. Such low demagnetisation factor N means that the effective magnetic permeability xcexcrxe2x80x2 at the a.c. frequency of operation is not reduced very much in comparison with the bulk permeability xcexcr and remains very high.
In a preferable embodiment of the invention, the magnetic material used as security feature comprises two or more types of magnetic material with different magnetic coercivity values or coercive forces, e.g. two or more different thin semi-soft magnetic films. In comparison with security features which only comprise a single semi-soft magnetic material with only one coercivity value, such a security feature with two or more different values of coercivity has the following advantages:
(a) it is easier to detect and to distinguish from other soft magnetic and semi-soft magnetic materials;
(b) it is more difficult to copy as security feature
(c) The detection algorithm is more difficult to copy.
In a preferable embodiment of the invention, the level of magnetic noise in the magnetic material is also detected. This level of magnetic noise is determined by measuring the variability of the electromagnetic detection signal. The noise is believed to be caused as the varying drive field causes discontinuous jumps in the magnetisation due to jumps in the positions of boundaries between adjacent domains. This phenomenon as such is generally known as the Barkhausen effect. The magnitude of the magnetic noise is dependent on the magnitude of the field and on the materials, grain sizes and geometry of the structure. It can therefore be used to identify particular materials and constructions.
The magnetic noise can be tailored by varying the thickness, composition and texture of both the underlayer and magnetic layer of the tag material. The texture of the underlayer, which depends amongst others on the thickness and composition, induces a texture in the magnetic layer that results in pinning centers for the magnetic domain walls. Moreover the small thickness of the magnetic layer results in considerable surface pinning effects for the domain walls. The underlayer and thinness of the magnetic layer combined results in a magnetic noise that can be tailored.
If the distance between the detector coils and tag varies, as would occur with a hand-held reader for example, then this would give a variation in the returned signal which could be confused with the effect of magnetic noise. We have found however that this effect can effectively be minimised by differencing subsequent readings of the returned signal amplitude in the following way.
Therefore the variability (V) is calculated from following formula:   V  =            ∑              i        =        1                    n        -        2              ⁢          xe2x80x83        ⁢                  [                              (                                          P                                  i                  +                  2                                            -                              P                                  i                  +                  1                                                      )                    -                      (                                          P                                  i                  +                  1                                            -                              P                i                                      )                          ]            2      
where n is the number of measurements of the amplitude of the electromagnetic detection signal (20).
Taking five (5) as the number of measurements, then P1 to P5 are five measured readings of the detection signal amplitude then a variability parameter can be calculated from:
V=((P3-P2)xe2x88x92(P2xe2x88x92P1))2+((P4-P3)xe2x88x92(P3-P2))2+((P5-P4)xe2x88x92(P4-P3))2
This calculation has the advantage of removing linear variations in the amplitude. In practice, with a hand-held reader it is found that a sampling rate for the signal can be determined which is slow enough to see the effect of the magnetic noise, but which is fast enough that the variation due to movement of the detector, relative to the tag, gives a closely linear relationship between the successive readings. It may be desirable to base the measurement on more or less than five readings.